The present invention is directed to a pigmented adhesive. More particularly, the present invention pertains to a pigmented adhesive for use with articles subjected to heat, such as hot stamp transfer labels.
Printed labels are well-known and commonly used to transfer a graphic, such as text or a figure, onto an item, such as apparel or merchandise. A label is usually pre-printed with a graphic, and then the graphic is transferred from the label to the item using a heated pad or iron or the like.
Printing techniques such as gravure printing, offset printing, flexographic printing, screen printing and digital printing all can be used to create a heat transfer label. Common to each of these processes is that the graphic is formed on a web or “label substrate” in which, generally, the label substrate is a substrate, such as paper or polymeric material, onto which a release layer is applied. The ink graphic is applied to the release layer, followed by an adhesive. Thus, the adhesive is applied to the top surface of the graphic. When a user then applies the graphic to the item, the label is turned adhesive-side down onto the item and heat is applied to the back of the label substrate to transfer the graphic to the item from the release layer of the label substrate.
Generally known ink formulations consist of resin or polyurethane dispersions, pigment dispersions, surfactant(s), and an amine, with a cross-linker added to the mix prior to coating. The adhesive, on the other hand, is generally formed from resin or polyurethane dispersion, surfactant(s), amine(s), and adhesive powder.
While the composition of ink and adhesive described above and the method of applying the graphic to items is effective, it is not efficient. Each layer of the graphic must be mixed and applied separately; the ink and the adhesive cannot be mixed together and applied in one stroke as the ink and the adhesive need to maintain different chemical characteristics.
For example, labels on garment must be stretchable, water wash resistant, and chemical dry-cleaning resistant. For these reasons, the ink used in the graphic must be cross-linked to form a 3-dimensional network, i.e. becoming a thermoset plastic.
In contrast, the adhesive cannot be cross-linked. The adhesive must be capable of being thermally activated and heat sealable in order for the user to transfer the graphic from the label to the garment. As the label is applied (i.e. hot stamped) the adhesive coated on the last layer of the label becomes “transferable”, meaning the adhesive softens and becomes tacky, capable of being adhered to the surface of an item. The ink layer adheres to the adhesive such that the ink graphic and the transferable adhesive together can be applied to an item. The adhesive cannot be cross-linked; as such, the ink and adhesive cannot be combined in one mixing pot as the mixture may result in the adhesive losing its thermoplastic characteristics.
Another drawback of printing a separate adhesive layer is the resulting border of adhesive which extends beyond the colored graphic. All printing processes have a “color to color tolerance” and the adhesive layer must be sized larger than the graphic to incorporate this color to color tolerance. This tolerance, which is determined by the amount of press movement, is inherent in the printing equipment. The adhesive layer is sized larger than the graphic to ensure that even when there is press movement there is a high level of confidence that the adhesive will be behind the entire graphic. For example, if the color to color tolerance of the press is ±0.005 inches, under optimum conditions there will be an even border of adhesive extending 0.005 inches around the entire graphic. Under some conditions, for example, there will be a border of adhesive extending 0.010 inches beyond the graphic in one direction and no border in the other direction. Even under these scenario conditions, however, there is still adhesive behind the entire graphic and the label can be successfully applied to the garment. The net result is that there is an adhesive border on the label which is visible on the garment as a halo around the graphic. This halo is clearly undesirable.
One technique to eliminate the halo effect is to use an adhesive powder. A solid powder consisting of a thermoplastic polymer which has thermal adhesive properties to various fabrics is spread over the wet ink of the graphic during the last step in the printing process. The inks are cured and the adhesive powder will only adhere to the ink with the extraneous powder being removed by either a vacuum process or some other mechanical means. The resulting label has the thermoplastic adhesive resin behind the printed graphic and there is no adhesive border or halo. Due to the size of the powder particles, however, there are some limitations to how fine a line weight can be practically covered with a powder adhesive.
Of course, whether the adhesive is a printed layer or a powder layer applied at the end of the printing process, there is a cost associated with the application of any adhesive to the label.
Accordingly, there is a need for a composition of ink and adhesive that minimizes the steps needed in creating a heat transfer-type label, prevents the premature cross-linking of the adhesive prior to application of the graphic, and prevents “haloing” of the adhesive. Desirably, such a composition allows for long-term storage of pre-made labels and such long term storage does not minimize or diminish adhesivity. More desirably, such a composition is easy to make, use, and apply.